This invention relates generally to apparatus and methods for making paper webs, such as webs for making facial tissue, bath tissue, paper towels, wipes, napkins and the like, and more particularly to apparatus and methods for applying a vacuum to the paper web during the making of such webs.
In conventional apparatus for making paper webs, a paper stock is fed onto endless foraminous belts or “fabrics” which are driven and supported by suitable drive rolls and tensioning rolls. The moving fabrics thereby serve as the surfaces on which the paper webs are formed while being transported in a machine direction by the apparatus. Typically, an aqueous suspension of papermaking fibers is delivered onto a first, or “forming” fabric to form a wet web which is then carried downstream past one or more vacuum devices, also commonly referred to as vacuum boxes. The vacuum devices apply a vacuum to the forming fabric and the wet fibers supported by the fabric to facilitate dewatering of the wet web. Additional dewatering may be accomplished by supplemental noncompressive dewatering techniques, such as infra-red drying, microwave drying, sonic drying, throughdrying, superheated or saturated steam dewatering, supercritical fluid dewatering and/or displacement dewatering.
The wet web is then transferred from the forming fabric onto another fabric, or “transfer fabric,” with the assistance of another vacuum device. For example, to transfer the web from one fabric to another, the transfer fabric is moved in opposed relationship with the forming fabric and passed over the vacuum device. The forming fabric, supporting the wet web in opposed relationship with the transfer fabric, converges with the forming fabric at the vacuum device whereby the vacuum device draws the wet web from the forming fabric onto the transfer fabric. The fabrics diverge from each other downstream of the vacuum device, leaving the wet web supported by the transfer fabric. Various apparatus and methods have been proposed that assist or facilitate the transfer of a paper web from a first fabric to a second fabric. For instance, U.S. Pat. No. 5,830,321 to Lindsay et al., which is incorporated herein by reference, discloses a method for improving the rush transfer of a wet paper web between two separate fabrics. The wet web is then carried downstream by the transfer fabric for additional processing in a conventional manner to form the desired end product. For example, U.S. Pat. No. 6,306,257, the entire disclosure of which is incorporated herein by reference, discloses one manner in which the web may be further processed following such a transfer.
The vacuum device used in the conventional paper making apparatus typically comprises a housing having a web-facing surface over which the wet web is transported by the fabrics, and a vacuum channel formed within the housing and open to the web-facing surface. A source of vacuum is in fluid communication with the vacuum channel to apply a vacuum to the wet web as the web passes over the web-facing surface (e.g., air is drawn into the vacuum channel at the web-facing surface). One drawback associated with the use of such a vacuum device for drawing a vacuum on a wet paper web is that the vacuum device often draws wet fibrous material from the web into the vacuum channel. The wet fibrous material has a tendency to adhere to the walls of the vacuum channel. Build-up of the wet fibrous material within the vacuum channel results in a decrease or loss of vacuum pressure. This requires the machine to be periodically shut down and the vacuum channels cleaned out.